The invention relates to a sheet separating apparatus provided with suction elements which are operated by means of air suction channels for suction attachment of the uppermost sheet of paper, cardboard or the like from a stack of such sheets and a subsequent sequence of a lifting of the suction element holding the sheet and a conveying-on of the sheet, which is thereby detached from the suction element, whereby the pressure drop in the suction channels caused during attachment of a sheet is used to introduce the lifting motion of the suction element holding the sheet.
This type of sheet separating device is known, for example from German Pat. No. 1,085,893. According to the structure disclosed by the German patent, an element of the device that draws a sheet and an element of the device that lifts this first device element with the attached sheet are united in a single piston-like structural element. The suction force of a vacuum source, which effects the suction attachment of the sheet simultaneously causes the lifting suction force for the piston-like structural element, so that the negative pressure that is sufficient for the lifting thereof is automatically set by the attached sheet after the closing of the suction openings. The lower side of a suction nozzle, which comes into contact with the sheet and is formed on the lower end of the piston-like structural element thereby contains at least two separate suction zones, the longitudinal bores of which each lead through the piston-like structural element. These longitudinal bores open on the end of the piston-like element opposite the suction nozzle in such a manner that at the end of the piston stroke the opening of at least one of the longitudinal bores is closed by the cover of the cylinder that guides the piston-like element, and simultaneously a cross bore that communicates with this longitudinal bore is aligned with a ventilation opening that is provided in the guide housing, so that a decrease of the suction force holding the sheet is produced, thus facilitating the removal of the attached sheet.
In devices of this type the piston-like element tends after a period of use to be difficult to move and to jam, because powder, paper dust, fibers and other impurities are aspirated with the sheet that is to be lifted from the stack. These impurities foul-up the piston-like element in the region of the suction nozzle both from the outside, so that the fouled portion travels into the cylinder guide during the lifting movement, and also from the inside, because the longitudinal bore of the piston-like element, in which the negative pressure must be maintained even after the lifting of the suction nozzle, that is, in the uppermost position, must remain in connection with a suction channel in the guide cylinder by means of an opening in the piston wall. The difficulty of operation that is thus produced after a period of time makes it requisite to frequently dismantle the device for cleaning purposes. Decreasing the suctional force which holds the attached sheet, in order to facilitate the removal of the sheet is possible only by an invariable amount; an adaptation to sheets having varying weights and different quality surfaces is not provided.
The basic purpose of the invention is to avoid as far as possible the fouling of the piston-like element of a lifting device in sheet separating devices, of the type mentioned at the outset, by aspirated particles and thereby also to avoid the consequences of such fouling, namely the sticking of the lifting device and wear of the sliding parts, thus making the device as nearly maintenance-free as possible. For sheet separating devices which are to be used in many ways--not just with folding machines--there are, however, a number of additional requirements, whose fulfillment should not be limited by the solution of the problem of avoiding the damaging results of aspirating fouling particles. Such items to be considered are the simplicity of construction, inexpensive production possibilities, rapid operation (which for use of the sheet separating device with a folding machine is very important), and also the effectiveness and adaptability of the devide to the handling of sheets of varying weights and surface qualities.
The solution to the problem according to the invention is that a separating device of the type mentioned at the outset is provided with a lifting device which has a separate energy supply from the suction device, but carries the suction device, and that the pressure drop that appears in the suction channels and suction lines when a sheet is drawn thereagainst is used to control the drive of the lifting device, preferably a pneumatic drive. By means of this separation of the effectiveness of the lifting device from that of the suction device, many advantageous constructional and switching possibilities are opened.
In one exemplary embodiment of the invention a vacuum source is connected as a pneumatic energy source both to (1) the suction line for the suction device that attaches the sheet, and (2) through a second suction line to a slide valve, which in turn acts upon a lifting piston for a carrying body of the suction device. For the control of the slide valve a control line branches off from the suction line for the suction device.
In another exemplary embodiment of the invention a vacuum source is connected to the suction line of the suction device as an energy source and a pressurized air source is connected to the slide valve as an energy source, with the slide valve serving to act upon a lifting piston for a carrying body of the suction device, and a control line for the control of the slide valve branches off from the suction line for the suction device.
A further development of this exemplary embodiment of the invention lies in the fact that at least one branch of the control line is led to at least one further control unit.
In still a further exemplary embodiment of the invention the lift displacement by which the suction device is lifted after the attachment of a sheet is divided into a preliminary stroke that comprises a fraction of the entire stroke and a main stroke, and the preliminary stroke is used to control the feed of the lifting device that causes the main stroke.
The invention will be better understood as well as further objects and advantages thereof become more apparent from the ensuing detailed description of three exemplary embodiments taken in conjunction with the drawings.